1. Field of the Invention
The present invention relates to an electric hammer drill having a dust collecting device for collecting duct produced during a drilling operation.
2. Description of the Prior Art
One known electric hammer drill of the type concerned is illustrated here in FIGS. 5 and 6. During a drilling operation using a drill bit 1, dust which is produced in the vicinity of the drill bit 1 is drawn by suction into a hood 2 of a vacuum dust collecting device. Subsequently, the dust is conveyed through a first pipe 3 connected at one end to the hood 2, through a second pipe 4 having one end slidably receiving the opposite end of the first pipe 3, and through a hose 5 having one end connected to the opposite end of the second pipe 4 and then is collected into a dust collecting chamber 6 defined in a body 7 of the hammer drill. Finally, the dust is discharged from the dust collecting chamber 6 into a dust bag 8 (FIG. 6) attached to the body 7. As the drilling operation proceeds, the body of the electric hammer drill moves forward. During that time, the hood 2 and the first pipe 3 retracts or moves backward. The second pipe 4 is fixedly attached to the body 7 via a side handle 9 and a holder 10. A compression coil spring 11 is disposed within the second pipe 4 and urges the first pipe 3 axially outwardly from the second pipe 4, so that the first pipe 3 is slidably movable relative to the second pipe 4 by and against the force of the spring 11.
The dust collecting chamber 6, as shown in FIG. 6, is defined jointly by first and second casings 12, 13 and houses a dust collecting fan 14 press-fitted to an armature shaft 15 of an electric motor 16 which is mounted in the body 7 for driving the drill bit 1 and the dust collecting fan 14. The armature shaft 15 extends across the dust collecting chamber 6 and is supported on opposite sides of the dust collecting chamber 6 by means of a pair of bearings 17. The dust collecting fan 14 is rotated by the motor 16 to draw the dust from the hood 2 into the dust collecting chamber 6 and then discharge the dust into the dust bag 8.
With this construction, the dust produced during a drilling operation tends to adhere to an inner peripheral wall of the second pipe 4 and the periphery of the spring 11, thereby hindering smooth sliding movement of the first pipe 3. In the worst case, the first and second pipes 3, 4 are locked together by the dust trapped therebetween. The conventional hammer drill is, therefore, unreliable in operation. Furthermore, due to the presence of the second pipe 4 extending between the first pipe 3 and the hose 5, the flow resistance of the dust collecting device is relatively large, and air may leak from a clearance between the first and second pipes 3, 4, thus lowering the dust collecting efficiency of the dust collecting device.
The armature shaft 15 extending across the dust collecting chamber 6 is relatively long and hence is likely to bend or flex during the drilling operation, causing an enhanced vibration of the body 7 of the electric hammer drill. Under such strongly vibrated condition, an armature winding of the motor 16 is likely to be damaged or otherwise broken. Since the dust collecting fan 14 and an end ring 19 are press-fitted to the armature shaft 15, they must be replaced together with the casing 5 when an armature 18 is to be replaced due to damage. The conventional electric hammer drill is, therefore, uneasy to maintain.